Jökull - 01.12.1979, Side 33

TYPES OF SUBAERIAL BASALT VOLCANOES IN ICELAND
Form of feeder channel Addenda
Eruption products Short fissure or tubular channel Long fissure
Decreasing temperature Lava Lava ring Type: Eldborg near Krýsuvík Lava lake usual in crater The lava flows are
Increasing explosivity Effusive activity i Lava shield Type: Skjaldbreidur | mainly pahoehoe (Icel. helluhraun)
prodi ra íction te Lava and tephra Agglutinate cone Type: Búrfell near Hafnarfjördur Crater row Type: Threngslaborgir The crater rows of mixed eruptions often develop cra- ters of both types,
r Mixed activity Scoria cone Type: Búdaklettur Crater row Type: Vikraborgir The lava flows are mainly aa (Icel. apalhraun)
Tephra Tephra cone Type: Raudaskál Tephra cone row Type: Vatnaöldur The volcanic activity is influen-
Phreato-magmatic activity Tephra ring Type: Hverfjall ced by contact of magma with | water
Maar Type: Grænavatn Explosion chasm Type: Valagjá
in historical time and are active in that sense, but it
is a rule with few exceptions that the Postglacial
Icelandic crater rows erupted only once and are not
active in the sense that they are likely to erupt
again. A parallel fissure may, however, open up a
short distance to the side of a crater row, but
whether or not this should be regarded as a sepa-
rate volcano is a matter of definition.
On the map Fig. 1, are listed all eruptions known
to have occurred in Iceland’s historical time. Be-
tween 30 and 40 volcanoes in the sense of eruption
site have been active during this period and an
eruption has started on average every fifth to sixth
year during the last three centuries. Before that
time a lot of eruptions were not recorded. The
eruptions are far from evenly distributed in time.
Between 1934 and 1961 only one eruption took
place, the Hekla eruption 1947/48, but the Askja
eruption in 1961 has been followed by the Surtsey
eruption 1963—67, eruption in Hekla 1970, in
Heimaey 1973, and activity in Krafla since 1975
resulting in 3 minor eruptions. This increase in
activity has coincided with increased activity on
the North Atlantic rift zone as a whole, starting
with the eruption off the coast of Fayal on the
Azores in 1957.
The majority of historical eruptions in Iceland
have occurred within the central volcanoes, three of
which — Hekla, Katla and Grímsvötn — have
been particularly active (Fig. 1). Occasionally
eruptions have affected the fissure swarms associ-
ated with the central volcanoes, resulting in small
or large scale rifting of the crust along these fissure
swarms. Out of about 20 eruptions reported per
century on average over the last three hundred
years, only 3—4 per century are of this type. The
reason is perhaps that a certain tensional stress
must accumulate across a swarm before it splits up.
A central volcano that is fed by a local magma
chamber may erupt several times before a stage is
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